This invention relates to a method of improving the optical, electrical, thermal, and mechanical properties of chemical vapor deposition (CVD) diamond. CVD diamond can be classified as either single crystal or polycrystalline. Either type can be manufactured to produce materials that range from opaque to fully transparent. Typical impurities within CVD diamond are graphite and hydrogen, although trace amounts of other materials may be present, such as nitrogen. In addition to impurities, there are structural defects which occur that further degrade the material and its properties as compared to a defect free natural diamond. As a result, CVD diamond is often opaque or very dark
Most industrial applications for diamond require high quality crystals or films. Common applications include lenses that require high optical transmission of light, heat sinks that require very high heat conductivity, and electrical insulators. Prior work to improve these materials by high temperature treatment has shown that heating above 850xc2x0 C. significantly degrades the sample. In fact, temperatures above 1600xc2x0 C. have totally destroyed sample integrity due to formation of cracks thought to be the result of loss of bonded hydrogen or conversion of the diamond carbon to graphite.
Natural or synthetic diamond, on the other hand, can withstand treatment to very high temperatures. In fact, it has been shown that annealing of synthetic and natural type I or type II diamonds in the range of 1900xc2x0 C. to 2600xc2x0 C. at pressures in the range of 50 to 80 kbars causes the visible color of the diamond to change. In the case of natural diamond type I, the color changes from brown to yellow or yellow-green. For type II natural diamond, the color changes from brown to colorless or, on rare occasions, blue or pink. Synthetic diamond will change from yellow to lighter yellow.
It would be advantageous if a method were devised that would significantly improve the properties of CVD diamond after it is grown. It would also be desirable to form CVD diamond with fewer defects that serve to degrade the intrinsic properties of a perfect crystalline diamond material in order to enhance its usage in many applications.
According to the present invention, there is provided a method of improving the optical properties of CVD diamond, which includes the steps of:
1. creating a reaction mass by placing the CVD diamond in a pressure transmitting medium that completely encloses the diamond; and
2. subjecting the reaction mass to a temperature of at least 1500xc2x0 C. and, preferably, in the range of about 1800xc2x0 C. to about 2900xc2x0 C. under a pressure of at least 4.0 GPA.
The period of time during which the sample is subjected to HPHT conditions is from less than about one minute to about 30 minutes. The preferred time is between one to five minutes. The actual conditions can be varied depending on the grade and the size of the CVD sample.
The reaction mass may be subjected to any number of such treatments. Thus, if the desired results were not achieved the first time, the sample may be re-treated at HPHT until such time that the desired improvement in characteristics or properties is achieved.
The greatest improvements will be noted for single crystal CVD diamond that is void of defects, such as surface pits, microscopic inclusions, and that is at least partially translucent. Such CVD material may be so improved as to even be polished and faceted to produce a gem quality diamond to be used in jewelry.